1. Field of the Invention
The present invention relates to a method of fabricating a ferroelectric liquid crystal display, and more particularly, to a method of fabricating a ferroelectric liquid crystal display having a micro-domain, which is adapted to have a bookshelf structure when crystallizing a ferroelectric liquid crystal, thereby increasing resolution.
The present invention is based on Korean Patent Application No. 2001-27272, which is incorporated herein by reference.
2. Description of the Related Art
A liquid crystal display is a flat type display, which is widely used for portable devices. Due to rapid development of display technology, the liquid crystal display is rapidly replacing a conventional CRT (cathode ray tube) display.
There are various kinds of liquid crystal materials that can be used in the liquid crystal display.
One type of liquid crystal display that is widely used is a TN (twisted nematic) liquid crystal display. The TN liquid crystal of this display utilizes the interaction between the dielectric anisotropy of liquid crystal molecules and an electric field, causing several drawbacks including an inefficient display of moving pictures due to a slow reaction time of a few tens of milliseconds (ms), and a narrow visual angle. Also, since cross-talk occurs between pixels within a certain distance, it is difficult to reduce a pixel size.
Another type of liquid crystal display is an FLCD (ferroelectric liquid crystal display). The FLCD utilizes the interaction between the spontaneous polarization of ferroelectric liquid crystal and an electric field, and provides a rapid response property of 1 ms or below to display moving pictures without any difficulty. An FLCD also provides a wide visual angle. The pixel size, in which cross-talk between pixels does not occur, can be reduced due to the strong interaction between molecules in an FLCD, so that high resolution display can be achieved. Because of the advantages described above, the field of FLCD has been researched extensively as a next-generation display device.
One widely used ferroelectric liquid crystal material is a chiral smectic C-phase (SmC*) liquid crystal material having a bi-stable property and a chevron structure.
In a fabricating process of the FLCD device using the liquid crystal material described above, the liquid crystal material is injected into a cell between substrates, while being maintained at a desired temperature, which is higher than the melting point of the liquid crystal material. Then, when the temperature is lowered, the liquid crystal material of chiral smectic C-phase (SmC*) is transformed to a chiral nematic phase (N*), and then to a smectic A-phase having a layer structure perpendicular to a rubbing direction, and then transformed back to the chiral smectic C-phase.
In this process, a long-axis direction of a liquid crystal molecule in a liquid crystal layer is tilted to a desired angle relative to the rubbing direction, thereby reducing the space between smectic layers. As a result, the smectic layer is bent in the liquid crystal layer in order to compensate for a change in volume. The bent layer structure is called the chevron structure, which has defined domains, each having a different long-axis direction according to the bending direction. On a boundary surface between the domains, there is formed a non-uniform orientation having a zigzag defect, a hair-pin defect, and a mountain defect.
Due to the non-uniform orientation property described above, a contrast ratio is remarkably lowered. If a DC voltage is applied to prevent the lowering of the contrast ratio, ions within the liquid crystal layer are accumulated on or adsorbed into a surface of an alignment film. Therefore, problems like afterimage effect occur. That is, the previous display pattern is dimly displayed on a current display pattern, when a previous display state is changed to the current display state.
Further, a ferroelectric liquid crystal material for providing an AFLC (anti ferroelectric liquid crystal) mode, in which the threshold limit is reduced, is actively being researched. However, since this ferroelectric liquid crystal material has a spontaneous polarization value of 100 nC/cm2 in AFLC mode, the ions in the crystal are moved due to a depolarization field, and thus an afterimage can be generated. Also, if an active matrix driving method, in which the liquid crystal is independently driven in each pixel using a TFT (thin film transistor), is applied, leakage current can be generated by the large spontaneous polarization value. In order to restrict the leakage current, a capacitance has to be increased. However, in this case, since an aperture ratio is reduced, it is difficult to use it as a display device.
Meanwhile, the ferroelectric liquid crystal material having a bookshelf structure, in which AC driving is possible and the afterimage is restricted, has been steadily investigated. However, since the ferroelectric liquid crystal crystallized by a drop in temperature displays black at a negative potential and displays white at a positive potential, when the AC driving is performed to prevent the afterimage, luminance is reduced to a half. Further, when applying the negative potential, black is not perfectly displayed due to a leakage light.
It is, therefore, an object of the present invention to provide a method of fabricating a ferroelectric liquid crystal display, which has high contrast, high resolution, and a symmetrical displaying characteristic with respect to an applied negative/positive potential.
In accordance with one aspect of the present invention, there is provided a method of fabricating an FLCD, comprising steps of: forming a lower structure and an upper structure, each having a structure in which a substrate, an electrode layer and an alignment film are sequentially formed; forming a cell for injecting liquid crystal on one of the lower and upper structures; bonding the lower and upper structures to each other; injecting ferroelectric liquid crystal through the cell formed between the lower and upper structures and then sealing the cell; and applying an AC potential of a desired frequency to the electrode layer, while dropping a temperature of the ferroelectric liquid crystal so that a phase of the ferroelectric liquid crystal is transformed corresponding to a bookshelf structure.
In one embodiment, the material of the ferroelectric liquid crystal has a property, by which the phase of the ferroelectric liquid crystal is transformed from a chiral nematic phase into a chiral smectic C-phase while dropping a temperature of the ferroelectric liquid crystal.